Windmills, and rotating turbines generally, have been used for grinding grain; pumping water; and, beginning in the twentieth century, as wind turbines to convert kinetic energy of wind into electric power. Traditionally, wind turbines used to generate electricity from wind are horizontal axis turbines with aerodynamic blades radially arranged around a horizontal axis and fixed to an axle mounted at the top of a vertical mast.
Over the years since their introduction, the size and capacity of horizontal axis wind turbines have increased in order to generate more power from a single mast, thus seeking economies of scale to reduce the cost of electricity produced over their lifetimes. The additional power produced by a larger horizontal axis wind turbine requires a generator of larger capacity that, with currently deployed generator technologies, results in a proportionally larger and heavier generator. Consequently, a significant increase in the capacity of the horizontal axis wind turbine requires that its components be significantly larger and be built to withstand significantly larger loads. For example, an increase in the size of the generator necessitates an increase in the length and overall dimensions of the entire blade structure to accommodate both the additional weight and the incremental forces to which the blade will be subjected. However, the blade length of horizontal axis wind turbines has practical limits. Also, increasing the length of the blades makes them more complex and expensive to manufacture as well as to transport, especially in the case of offshore assemblies which will require large seagoing cranes for installation, maintenance and retrofitting of the generator.
Increases to the size of wind turbine generators will also complicate the assembly of those generators. At present, conventional generators require that the wind turbine components be lifted by cranes for placement and securement to a vertical mast via the peak of the mast. This is the case in both horizontal axis turbines, where the wind turbine sits at the peak of the mast, and vertical axis wind turbines, where the wind turbine is lifted above the mast peak and then lowered down to insert the mast peak through an opening in the wind turbine. As such, in either axis type, increases in generators size will require ever larger cranes and lifting equipment to raise the wind turbine units for mounting to the mast via the mast peak.
Increases in generator size also introduce further complications concerning component failure and maintenance. Larger generators will generally require more components and practically all components of a wind turbine constitute a single point of potential operational failure. That is, if one component in the turbine fails, the entire turbine ceases to generate electricity. The chances of component failure are enhanced when the wind turbine is exposed to environmental conditions prone to icing. Even in the absence of component failure, larger generators with more surface area for ice accumulation will present increased maintenance concerns. The accumulation of ice on the blades decreases aerodynamic efficiency and consequently the turbine's power conversion capacity. Excessive ice accumulation can also lead to dynamic loads greater than those tolerable according to design specifications requiring the operator, in these cases, to stop the turbine in order to prevent damage.
Accordingly, there remains a need in the art for a turbine system of a robust construction that can generate greater magnitudes of electrical power while also withstanding the increased stresses that accompany the increased output. There is also a need for a turbine system that facilitates accessibility, maintenance, and replacement of individual components—and does so without requiring a shutdown of the system while maintenance and repairs are underway, such that the system may continue generating electric power, even if at a lower generation capacity. There is also a need for a turbine system that may be assembled with commercial construction equipment—especially in offshore applications, such that there may be avoided any need for large seagoing cranes of considerable expense. There is also a need for a system that inhibits excessive ice accumulation.